Coffeehouse Thread

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In the name channel9.msdn.com ...

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  • User profile image
    Minh

    In the name channel9.msdn.com, I know that msdn.com is called the domain name. But what is the channel9 part called.

    Thanks.

  • User profile image
    gizmo_

    Subdomain i think...

  • User profile image
    Minh

    Makes sense Smiley

  • User profile image
    Sven Groot

    Note that according to RFC1034, which defines the domain name system, not only is channel9 a subdomain of msdn, but msdn is a subdomain of com, and com a subdomain of the root domain.

  • User profile image
    Sven Groot

    Maurits wrote:
    channel9.msdn.com = FQDN (Fully Qualified Domain Name)

    Actually, no. A FQDN must end in a dot.

  • User profile image
    Maurits

    com = TLD (Top Level Domain)
    msdn.com = domain name
    channel9.msdn.com. = FQDN (Fully Qualified Domain Name)
    channel9 = Host Name or Alias, depending on whether an A record is used (host) or CNAME record (alias)
    In this case, an A record so it's Host Name

    EDIT: fixed FQDN (thanks Sven)

  • User profile image
    Maurits

    www.microsoft.com, on the other hand, is a CNAME or alias for www.microsoft.com.nsatc.net

  • User profile image
    Minh

    Maurits wrote:
    com = TLD (Top Level Domain)
    msdn.com = domain name
    channel9.msdn.com. = FQDN (Fully Qualified Domain Name)
    channel9 = Host Name or Alias, depending on whether an A record is used (host) or CNAME record (alias)
    In this case, an A record so it's Host Name

    EDIT: fixed FQDN (thanks Sven)


    So a "host" indicates an actual computer? So how is "alias" different? What actually goes into a CNAME record?

  • User profile image
    Maurits

    Host records are looked up by name, and point to IP address
    Alias records are looked up by name, but point to other names

    www.microsoft.com is a CNAME (alias) for www.microsoft.com.nsatc.net
    www.microsoft.com.nsatc.net is an A (host - A stands for address) whose IP is 207.46.199.30

    I find Squish's DNSCheck utility very useful for troubleshooting DNS issues from a remote POV - Microsoft's nslookup and Linux's dig commands are also nice for local troubleshooting.

  • User profile image
    AndyC

    Minh wrote:


    So a "host" indicates an actual computer? So how is "alias" different? What actually goes into a CNAME record?


    An A (host) record maps a name onto an IP address, for example www.microsoft.com.nsatc.net onto 207.46.199.120

    A CNAME (alias) record maps a name onto the real DNS name, for example www.microsoft.com onto www.microsoft.com.nsatc.net

  • User profile image
    Minh

    Gotcha. Thanks all. I'm reorganizing my server. Off of my domain name, I have a SubDomains directory. Just wanted to name it correctly.

  • User profile image
    Maurits

    One common DNS configuration error is CNAME loops...

    larry.example.com CNAME to curly.example.com
    curly.example.com CNAME to moe.example.com
    moe.example.com CNAME to larry.example.com

    CNAME loops are bad, m'kay...

  • User profile image
    AndyC

    It always good practice to point CNAME records at A records rather than other CNAME records. It avoids the nasty loop problem, keeps the number of queries to a minimum and reduces the chance of an obscure breakage if you decide to move a service.

  • User profile image
    Sven Groot

    Beer28 wrote:
    apache has this feature, where  2 or more domain names can be pointed to the same IP address, and by the http "Host: example.com" parameter in the http headers, will redirect the request to a specific virtual site.

    This way you can have 1ip with 10,000 virtual hosts, instead of 1 per ip.

    Freeing up your subnet of ips.

    I don't think there's webservers out there that don't have this feature. IIS has definitely had it for a long time. Heck, I wouldn't be surprised if good ol' Personal Web Server could do it (okay, probably not).

  • User profile image
    Maurits

    That explains the dropoff in popularity of IE 3.0, which didn't bother to add the Host: header Wink

  • User profile image
    Maurits

    You can do that with pretty much any DNS server software.  Just configure a .beer28 zone, and set all the default domain names on the clients to beer28. The DNS server will be able to consult itself for zones it hosts, and will use the global DNS server system for all other zones.

    I use BIND and Microsoft DNS.

    Another advantage to running your own DNS software is you can disable domains that upset you.  Suppose you're feeling anti-Google one day - just declare a zone for google.com (and any other associated domains you like) and your LAN will suddenly be unable to resolve www.google.com.

  • User profile image
    Maurits

    DNS servers query the root servers directly regardless of what you put as the "DNS server" entry in your IP configuration.

    You can configure DNS servers to query other DNS servers - this is called "using forwarders."  For example, you could tell your DNS server to just ask your ISP's DNS server for everything it doesn't already know about.  But this is configured in the BIND confs, not on eth0.

    EDIT: after reading your post:

    I assume you have a router at 192.168.0.1 (say) which is serving DHCP.
    Your DHCP clients pick up your ISP's DNS settings over DHCP - your ISP's DNS server is, say, 10.1.2.3 (not really, just as an example)

    Suppose you set up a local DNS server at 192.168.0.2
    Configure all your LAN machines to use 192.168.0.2 as their DNS server - including 192.168.0.2
    Then your DNS server will be used by all LAN machines. Client processes on the 192.168.0.2 will effectively consult localhost.  The DNS Server process on 192.168.0.2 is smart enough not to consult itself - it will consult the root servers.
    OPTIONALLY configure your DNS Server process to consult 10.1.2.3 - using forwarders - if you like.  Might be faster.  It's probably to your advantage to leverage the large DNS cache of your ISP's name server.

  • User profile image
    Maurits

    Beer28 wrote:

    I'll have to see if there's a way to specify a seperate DNS server for the DNS service apart from the normal configuration which gets it from the router.


    Ah I see...
    The short answer is you don't have to.  Every DNS server software comes with a "root hints" file, containing the IP address of all the global root name servers

    The long answer is, you CAN, if you like.  Usually this is for cache leverage, or for very large organizations with heirarchical private domains.

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